In many circuit applications it is necessary to achieve AC coupling of signals. This may be necessary to keep the signal centered within the allowable voltage swing range to prevent clipping and/or to eliminate any unwanted DC component of the signal. Typically, elimination of the DC component is done by using a coupling capacitor feeding into a resistive load. This does not achieve centering of the signal swing ahead of the capacitor. However, for very low frequency coupling, the resistor and/or capacitor must be large. This in itself may prevent practical integration on an IC. In addition, the required level of residual DC offset for stringent applications, such as in a Direct Conversion baseband IF, may be so low that the required level cannot be achieved by conventional active circuit techniques.
In some applications it has been necessary to add coupling capacitors between V.sub.out and the input to the up mixers in order to eliminate the offset of the error amplifier to insure that the DC offset is below about 0.1 mV. In such applications, the offset correction loops keeps V.sub.out centered at analog ground to prevent signal clipping, but the residual offset is not low enough to avoid the need for the coupling capacitors.
In yet other applications, the problems are solved by combining the use of coupling capacitors to eliminate the DC component at the output, plus an offset correction circuit ahead of the coupling capacitors to maintain signal swing centering. Two long time constants are generated with that approach (the coupling capacitor time constant and the offset correction time constant) each of which may be a source of unwanted transient responses when the baseband DC level changes due to varying signal levels, etc.
It is, therefore, desired to have an electronic circuit with DC blocking that is independent of offset voltages that are inherent in integrated circuits.